The present invention relates to the field of signal processing systems. More particularly, the present invention relates to the field of signal processing systems for use in optical information recording systems. Specifically, the present invention relates to a signal processing system for overcoming frequency dependent modulation degradation as a result of limited resolution recording optics and other elements of the recording system.
A great many systems are known for optically recording information on light responsive media. Digital information as well as analog information can be recorded on a variety of media, including photoresists, photochromic materials, and thermally responsive materials in the form of discs, drums, and tape.
Virtually all of these known systems employ some form of pulse code modulation (PCM) or frequency modulation (FM) due to the nonlinearities of known optical recording media. Additionally, virtually all of these known systems employ an optical system including an objective lens for focussing the writing beam of light to a small spot on the light sensitive surface of the medium. In order to maximize the information density of the recorded material, it is necessary to focus the writing beam to the smallest possible spot size. Due to defraction effects, a focussed beam of coherent radiation forms a central bright spot surrounded by concentric spaced rings of light known as an Airy disc and having an approximately Gaussian power distribution. The diameter of the central spot of light is defined by the wavelength of the recording light and the numerical aperture (NA) of the objective lens.
Most optical recording media exhibit a threshold effect, meaning that a level of writing beam power density exists above which the medium will be altered, and below which the medium will not be altered. Since the power density of the focussed writing beam varies as a function of the distance from the center of the central spot, it is possible to form altered areas on a medium which are smaller than the diameter of the central spot of the focussed light. In order to obtain an altered area of a specific size, the prior art has taught that it is merely necessary to adjust the peak intensity of the modulated beam such that the power density of the focussed spot at the specified diameter is equal to the threshold level of the recording medium. Experimental results indicate that this technique works extremely well when there is no relative movement between the writing spot and the medium and no thermal conductivity effects diffusing the energy absorbed by the medium. In all practical systems, however, relative movement between the medium and the recording spot is necessary. This relative movement greatly complicates the calculation of the amount of energy absorbed by a particular area of the medium.
All light responsive media exhibit a tendency to integrate the amount of energy received over a period of time, so that the threshold level of the material does not define a specific instantaneous power density at which the material is altered, but rather defines an energy density which is the result of an integration of the received power over the time during which the power is received. In other words, the threshold level for the materials is actually an exposure level. Exposure level is defined as the radiant flux per unit area integrated over the exposure time. The radiant flux irradiating a particular point on the medium at a particular time is a function of the position of the point within the focussed spot, as well as the instantaneous modulation level of the writing beam. Specifically, the exposure level at a particular point on the medium is equal to the convolution integral of the instantaneous power of the modulated writing beam as a function of time and the power density distribution function defined by the path and speed of the particular point of the medium through the focussed spot, evaluated over the exposure time. Since the power distribution of the focussed spot varies in two dimensions, the exposure level over the medium defines a three dimensional graph. The intersection points of this graph with a planar "slice" through the graph at the altitude corresponding to the threshold level of the medium maps the boundries of the altered areas on the medium.
An important concept in determining the effect of the finite size of the recording spot is the concept of the spatial frequency of the information on the moving recording medium. The spatial frequency is the number of cycles of modulation of the writing beam seen by a unit length of the medium in the direction of movement of the medium. Naturally, higher frequencies of modulation and slower relative speeds between the spot and the medium increase the spatial frequency of the information. Since the area of the writing spot is fixed by the wavelength of the writing beam and the numerical aperture of the lens, the effects of the finite spot size increase with increasing spatial frequency.
The modulation transfer function (MTF) is a curve used to describe the ratio of object contrast to image contrast for a given optical system at various spatial frequencies of object information. The MTF at a particular spatial frequency is dependent solely upon the wavelength of the projected light and the numerical aperture of the optical system. The MTF is normally used in reference to imaging systems and can be measured for a particular system by using an object having a known spatial frequency, such as parallel lines which vary in intensity in a sinusoidal fashion of known spatial frequency and modulation level. An image of this object is formed using the optical system. The MTF is then the ratio between the modulation level of the object and the modulation level measured at the image plane of the optical system.
The prior art has recognized that the MTF of a lens system used for optically reading information from a storage member defines the recovered modulation amplitude in terms of the spatial frequency of the recorded information. When the information was recorded in such a manner that its spatial frequency varied, such as when the information was recorded on a disc turning at a constant angular velocity, the prior art recognized that the modulation level of the recovered information would vary with the change in spatial frequency of the information resulting from the change in speed of the medium relative to the reading beam. A closed loop system for correcting the modulation level of an FM signal recovered from a video disc turning at constant angular velocity is disclosed in U.S. Pat. No. 4,370,679, entitled "Gain Correction System for Video Disc Player Apparatus". The system disclosed in this patent varies the gain of the higher frequency components of the recovered signal by an amount proportional to the difference between the amplitude of a portion of the recovered signal and a reference amplitude. Other prior art systems are known which vary the gain versus frequency characteristics of the playback electronics as a function of the radial position of the reading beam on an information carrying disc.
It has been hithertofore unrecognized that the size of the recording spot as defined by the numerical aperture of the recording objective lens and the wavelength of the recording light not only affects the size of the information which can be written on the disc, but also causes a spatial frequency dependence in the modulation amplitude of the exposure level of the medium. The resulting decrease in modulation amplitude at higher spatial frequencies results in decreased operating level tolerances at higher spatial frequencies which can result in dropouts of high frequency information or at relatively slow relative speeds between the writing spot and the medium, even though the unfocussed beam is properly modulated and substantially perfect recording occurs at slightly lower spatial frequencies.
Accordingly, a need exists for an information processing system capable of compensating for the spatial frequency dependence of the exposure level modulation amplitude in optical recording devices.